Normally I would say a networking issue… but then I wouldn’t expect to see jumps in JME’s tpf like that.
The other thing would be GC as folks have mentioned… but GC shouldn’t be an issue in this app and others seem to run it fine, I guess. (Some even base whole games on this starting example.)
Next thing I guess is just to verify that something else isn’t stealing CPU from your app periodically. As a last resort, you can try rebooting the OS or something to see if that fixes it.